This invention relates to thyristors of the type employing photogeneration to provide gate current, such thyristors being generally known as light-fired thyristors.
Light-fired thyristors are particularly useful in controlling high voltage circuits where a high degree of electrical isolation is important. High voltage circuit control can be achieved from a relatively distant location through use of conventional light fibers to carry triggering light pulses from a light source to the thyristor. The light fibers employed have diameters as small as two or three thousandths of an inch. To enable the fibers to carry sufficient light to trigger a thyristor, numerous single fibers are bundled together to form a larger light conduit which is tied directly to the thyristor.
Even using numerous light fibers, the amount of available gate current supplied to the thyristor through photogeneration is low. Gate sensitivity must therefore be high. Typically, light-fired thyristors have relatively short turn-on lines; the turn-on line being the portion of the emitter-base junction closest to the source of gate current. A short turn-on line yields improved gate sensitivity but also reduces the available turned-on junction area when the thyristor begins to conduct. As a result, prior art light-fired thyristors are susceptible to premature failure resulting from excessive current gradients (di/dts) during initial turn-on.